The present invention relates to a magnetic recording disk drive. More particularly, the present invention relates to a technology for preventing the deterioration of the head off-track characteristics resulting from the asymmetry of erase band widths which occurs when an angle (hereinafter referred to as a “skew angle”) between a relative movement direction of a magnetic head, which executes the data write/read operations, with respect to a recording medium for storing information and a direction of magnetization reversal region formed on the recording medium is not 0 degree.
FIG. 2 shows an observation result of a recording state of information written on a disk by a head, through a magnetic force microscope. Here, data 1 is written on a data 3 written as an old information. As shown in FIG. 2, erase bands 2 (areas for erasing the old information, though new information is not written thereto) exist on the sides of the track for the recorded data 1. In a state in which the head is positioned to a target track (so-called “a following state”), the adjacent tracks are not erased. However, the erase bands 2 erase the information of the adjacent tracks when a write operation is performed before a residual vibration caused by a seek operation (i.e., a settling vibration) is not sufficiently settled immediately after the head is moved from another track, or when the write operation is performed in a state in which a large offset is caused by an external shock. In order to prevent such an accidental data erasure, a conventional magnetic disk drive is generally so constituted as to stop the write operation when an offset greater than a write inhibit slice value determined by the device occurs with respect to a target write track based on position signals acquired every moment.
FIG. 3 illustrates a rotary actuator system which is a predominant system in the existing magnetic disk drives. In the rotary actuator system, an actuator 6 which supports a head 5 rotates with point “A” in the drawing as the center, so that the head 5 moves between tracks in a disk 4 for storing information. As shown in FIG. 4, in the rotary actuator system, angle φ between a relative movement direction of the magnetic head 5, which writes or reads the information, to the disk 4 for storing the information and the direction of a straight line, which connects the device position of the head 5 to the rotary actuator rotation center “A”, changes as the head 5 moves from the inner periphery towards the outer periphery. In consequence, as is shown in FIG. 5, angle θ (hereinafter referred to as “skew angle”) between a disk travelling direction and a reversal region of magnetization 9 which is formed on the disk 4 changes as the head 5 moves from the inner periphery to the outer periphery. A report has been made to the effect that the presence of this skew angle renders the erase bands asymmetry (K. Wiesen et al., IEEE Trans. Magn., Vol. 29, pp. 4002-4004, 1993).
FIG. 6 schematically illustrates a recorded state on a disk when the information is written to the data track and adjacent tracks where the skew angle is 0 degree. It is assumed that the information is written first to the data track, then to the inner adjacent track, and finally to the outer adjacent track. No difference occurs in the width of the regions erased at the left and right ends of the data tack when the track effectively written has width Tww and the erase band widths on the right and left (which are equal on the inner and outer peripheral sides) are EB as shown in FIG. 6.
FIG. 7 schematically illustrates a recorded state on a disk when the information is written to the data track and adjacent tracks in the same way as in FIG. 6, on the assumption that the skew angle is not 0 degree but takes a finite value. It is assumed also that the information is written first to the data track, then to the inner adjacent track, and finally to the outer adjacent track, in the same way as in FIG. 6. The effective recorded track width is “Tww(s)”. The erase band width on the inner side is “EB(s)in”, and the erase band width on the outer side is “EB(s)out”. FIG. 7 shows the state where the erase band width EB(s)in is greater than the erase band width EB(s)out. As shown in FIG. 7, when the information is written at positions having a track pitch spaced apart by Tww(s)+EB(s)out on the adjacent tracks of the data track, the erase band of the outer adjacent track is in touch with the data track region of the data track. Therefore, the data of the data track is destroyed by the outer adjacent track if the track pitch becomes smaller than this track pitch. However, the data in the inner adjacent track, in which the data is recorded with the same distance, is not yet destroyed in the data track region of the data track. It can be thus appreciated that if the inner and outer erase bands of the data track are asymmetric, the recorded data track is first destroyed from one of the adjacent tracks with the decrease of the track pitch.
A 747 curve (J. K. Lee et al., IEEE Trans. Magn., Vol. 26, pp. 2475-2477, 1990) is affected by the presence/absence of the asymmetry of the erase bands. FIG. 8 shows an example of the calculation of the erase band asymmetry dependence on the 747 curve using an algorithm described in article “F. Tomiyama et al., IEEE Trans. Magn., Vol. 34, pp. 1970-1972, 1998”. It can be appreciated that the off-track capability reduces in a region having a small track pitch due to the presence of the erase band asymmetry. This results from the fact that a greater one of the right and left erase bands squeezes the adjacent track and starts erasing a part of the information written on the adjacent track.